This invention relates to a gravity settling apparatus and method for recovering a clarified liquid from a slurry comprising a liquid and suspended solids. In a particular aspect, the invention relates to the design of internals for a gravity settler.
The need for an effective and economical apparatus and method for removing suspended solids from a liquid slurry containing them is a continuing problem in the field of solid-liquid separation. The need is especially noticeable in the case of synthetic hydrocarbon liquid processing, such as coal liquefaction, tar-sand oil extraction and shale pyrolysis. Coal liquefaction extracts, shale oils and the like usually contain appreciable amounts of finely divided suspended solids. These solids can seriously interfere with catalytic processing for upgrading these oils to useful fuels due to catalyst deactivation by poisoning active sites, pore plugging, reactor plugging, etc. Typically, all suspended, finely divided solids have average diameters in the range below 100 microns.
These solids are commonly described in the art as unfilterable because, as a practical matter, satisfactory separation of them from the oil cannot be accomplished by usual mechanical separation techniques such as filtering, centrifuging and settling. Gravity settlers are used to separate solids from a slurry of the solids and the liquid, and they are especially useful in the application of separating coal oil from coal fines and ash. Examples of conventional gravity settlers include the following:
U.S. Pat. No. 4,075,080, which describes a process for separating undissolved solids from coal oil. The gravity settler described therein has a settling zone above a washing zone in which descending agglomerated solids are washed before they are withdrawn.
U.S. Pat. No. 3,791,956, which describes a settler comprising an upper quiet zone and a lower agitated zone. The feed is discharged downwardly into a central vessel, clarified liquid is withdrawn from an edge of the quiet zone, and solids are withdrawn from the bottom of the agitated zone.
U.S. Pat. No. 4,096,051, which describes a separator having a central attrition zone surrounded by a sand separation zone and an outer product recovery zone.
Settlers incorporating other arrangements of internals are shown in U.S. Pat. Nos. 3,184,065 and 1,104,051. In spite of the long history of the use of gravity settlers, there is still room for improvement in their design.